The present invention relates to a joint prosthesis which is in the form of a shaft and which is constructed as a hollow body, and comprised of perforated sheet metal and having a collar and a projection onto which the joint is set.
In endoprosthesis there is currently a differentiation between two variations of attachment of the prosthesis shaft in the bone. The prostheses are either attached by means of a bone cement in the prepared bone cavity or by using so-called cementless prostheses.
As a rule, good initial success is obtained with the cementing technique, as the cement adapts very well to the prepared bone cavity. However, since the bone cement becomes brittle after a certain amount of time, the prostheses become loose and have to be removed.
It was attempted to counter this disadvantage with the development of the cementless prosthesis. In the cementless technique, prosthesis shafts having a surface structure are inserted into the bone cavity which has been prepared as precise as possible and are fixed there by means of pressure setting. In order to obtain a permanent fixation of the prosthesis, the bone has to grow into the surface structure. However, as the flexural strength of the protheses shaft is considerably higher than the flexural strength of the bone into which it has been implanted, relative movements occur on stress between the bone and the shaft, which leads to a connective tissue separation of the prosthesis shaft and thus a loosening.
A prosthesis is already known, as described in DE-OS No. 31 20 147, which has a shaft that is designed as a hollow body, comprised of perforated sheet metal and having a collar and a protrusion serving for the attachment of the ball joint. It is possible to introduce into the inside of the hollow prosthesis, after the primary fixation, cut up bone chips, which accrues in sufficient quantity during the operation. The hollow shaft of the prosthesis is provided with a sufficient number of various size holes so that the bone chips pressed into the shaft can come into contact with the wall of the bone cavity and grow onto it.
An essential difference to the current cementless prosthesis technology thus resides in the fact that the bone does not have to grow into the surface structure. It is much rather possible with this construction that, already during the operation, the bone is brought to the place where it is needed. By the adequate sizing of the holes in the prosthesis shaft it is possible to take care that the bone inserted into the shaft grows on and can be nourished permanently.
The design of the prosthesis shaft as a perforated hollow shaft also has the advantage that with the arrangement and sizing of the holes, the flexural strength of the shaft can largely be adjusted to the flexural strength of the bone, so that during stress of the system the bone/prosthesis relative motions are slight.
However, the perforated hollow prostheses such as described in DE-OS No. 31 20 147 cannot be manufactured with defensible technical expenditure.